The principal aim of this study is to investigate the mechanism which underlies arrythmogenic spontaneous activity induced by cardiotonic steroids in cardiac muscle. It is proposed that in cardiac muscle preparations, digitalis intoxication promotes ectopic foci by inducing a novel transient inward current (TI). It is further hypothesized that phasic release of Ca ions from internal stores regulates the TI as well as activation of contraction. The experimental approach is to correlate simultaneous measurements of tension, membrane currents, and optical signals to link electrical and mechanical events to phasic changes in Cai. Shortened calf or rabbit Purkinje fibers will be voltage clamped using the two microelectrode method. Tension will be measured with a piezoelectric force transducer.